Solder is a material that typically contains tin and lead and that is commonly used during the manufacturing of electronic circuit boards. Solder generally has a lower melting temperature than the metals that may be included, as lines or layers, in the circuit boards. Hence, once two or more metal lines or layers have been formed in a circuit board, solder may be used to form an electrical contact between the layers and/or lines.
FIGS. 1A–1E show cross-sections of semiconductor device structures after various steps of a process for depositing solder on a planar semiconductor substrate surface have been performed according to the related art. FIG. 1A is a cross-sectional view of a semiconductor substrate 100, such as silicon or gallium arsenide, and of an organic film 110, such as a photoresist film, that has been deposited on the semiconductor substrate 100. The organic film 110, according to the related art, is typically spun onto the substrate 100 and is typically in contact with the entire surface of the semiconductor substrate 100.
FIG. 1B is a cross-sectional view of the layers 100, 110 discussed above after the organic film 110 has been selectively etched to form a series of holes 120 (or channels, troughs, grooves, or openings) above the substrate 100. The holes 120 in the organic film 110 may be formed via photo-lithography or by any other process known in the art of semiconductor device manufacturing.
FIG. 1C is a cross-sectional view of the substrate 100 and organic film 110 discussed above after the holes 120 in the organic film 110 have been filled, at least partially, with solder paste 130. Solder paste, in general, typically includes an admixture of flux and solder particles. The solder paste 130 shown in FIG. 1C may be deposited in the holes 120 by any process known in the art. For example, a process similar to the stencil printing process used in the surface mount assembly process can be used. Specifically, a squeegee can be used to “roll” a bead of solder paste 130 across the organic film 110 to deposit the solder paste 130 into the holes 120.
FIG. 1D is a cross-sectional view of the substrate 100 after the solder paste 130 has been heat-treated to form solder bumps 140 on the substrate 100. In order to form the solder bumps 140, the temperature of the solder paste 130 that had been in the holes 120 of the organic film 110 was raised. The higher temperature caused the flux portion that had been in the paste 130 to liquefy and activate the metal surfaces and caused the solder particles in the paste to melt. In the molten phase, the solder will wet to a solderable pad on the substrate surface while the surface tension of the liquid solder will cause the molten solder to form the shape of the solder bump. Upon cooling of the melted solder particles, solid solder bumps 140 were formed. Typically, the temperature of the solder paste 130 is raised by the use of an oven or hot plate.
FIG. 1E is a cross-sectional view of the substrate 100 and the solder bumps 140 after the organic film 110 has been removed. The organic film 110 may be removed by any process that known in the art. Upon removal of the organic film 110, the substrate 100 may have additional structures, such as metal layers and metal lines, deposited thereon, and the solder bumps 140 can be used to electrically connect two or more metal layers or lines.